Cellular communication systems are being developed and improved for machine type communication (MTC), communication characterized by lower demands on data rates than for example mobile broadband, but with higher requirements on, for example, low cost device design, better coverage, and an ability to operate for years on batteries without charging or replacing the batteries. In the 3GPP GERAN specification group, cellular communication systems are being improved and developed in the feasibility study named “Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things.” GSM is being evolved, and new “clean slate” systems, (systems not based on current cellular systems, are being developed.
One “clean slate” solution, called narrowband machine-to-machine (NB M2M), is a narrowband system with a carrier bandwidth of 200 kHz that targets improved coverage compared to GSM systems, long battery life, and low complexity communication design. One intention with this solution is to deploy it in spectrum that is currently used for GSM, by reducing the bandwidth used by GSM and deploying NB M2M in the spectrum that becomes available. Another intention is to reuse existing GSM sites for the deployment of NB M2M.
In cellular communication systems, devices use a cell search procedure (or synchronization procedure) to understand which cell(s) to connect to. A cell search procedure typically includes detecting a suitable cell to camp on, and for that cell, obtaining the symbol and frame timing and synchronizing to the carrier frequency. The cell search procedure for NB M2M is described in GP-140864, “NB M2M—Cell Search Mechanism,” and GP-140861, “NB M2M—Frame Index Indication Design.”
After switching on, an MTC device first needs to search for a signal in a viable frequency band. Signal detection is performed on the basis of comparing the amplitude of the peak from a correlation based detector with a pre-determined threshold. This is achieved by correlating the received signal with a known sequence, or a set of known sequences. In NB M2M systems, in order to fulfill the requirements of extended coverage, the cell synchronization procedure needs to be operable at very low signal to noise ratios (SNR).
In order to achieve this functionality, existing approaches for estimating frame timing, frequency offset and other relevant quantities require the accumulation of the processed correlator output over multiple frames, in order to obtain a sufficiently high peak. There are two separate sequences, namely the cell identification sequence (CIS) to detect the cell ID and the frame index indication sequence (FIIS) to obtain the frame number.
A problem with the typical NB M2M approach for cell synchronization stems from the design of the CIS and FIIS. The CIS sequence used is unique to a particular cell, while the FIIS is determined by two sequences: one sequence is used for the frame number of the current frame, and this is then scrambled with a cell specific scrambling sequence in order to provide protection from inter-cell interference. As a result, detection of the frame number requires knowing the cell specific scrambling sequence, which in turn requires knowing the cell ID. Therefore, in order to obtain the cell ID as well as the frame number, a sequential detection needs to be performed (i.e., the cell ID is obtained first, which is then used to detect the frame number). This increases the synchronization time, which may not be desirable.